Usually soldiers would attach bayonets before initiating a charge or entering combat, but it would be infuriating if you had to do that every time one entered combat in-game. I would think that this is one scenario where gameplay trumps real life, so expect the same thing as NW.

I'm curious as to what the downside would be (On the players' side, anyways. It'd definitely involve more work for the devs) to having fixable/removable bayonets. If you want it to be like NW, just fix it when you spawn, and away you go.